This invention relates to method of manufacturing hollow resin molding. A manifold made of synthetic resin used for an intake manifold of an automobile is a kind of hollow resin molding. Such manifold is formed by integrating a gas sucking portion, cylinder head connecting portions, and branch pipes for connecting both the gas sucking portion and the cylinder head connecting portions together. The manifold has therefore a complicated shape. Various manufacturing methods are available.
As shown in FIG. 15, a first method involves the steps of: forming a core 101 having the shape of a hollow portion of a manifold using a low melting point alloy; molding a manifold 103 made of synthetic resin over the outer circumference of the core by injection molding; and melting the core 101 made of the low melting point alloy thereafter to obtain the manifold having a predetermined shape.
As shown in FIG. 16, a second method involves the steps of: integrally molding an internal molding 105 having the shape of a hollow portion of a manifold by blow molding; reinforcing the internal molding by charging sand, water, or the like 107 thereinto; and integrating an external molding 109 over the outer circumference of the internal molding by injection molding to obtain the manifold with the predetermined shape.
As shown in FIG. 17, a third method involves the steps of: integrally molding an internal molding 111 having a predetermined shape of a hollow portion of a manifold by blow molding; and integrating an external molding 113 over the outer circumference of the internal molding by reactive injection molding to obtain the manifold with the predetermined shape.
As shown in FIG. 18, a fourth method involves the step of joining molding elements 115, 117 having a predetermined shape of a manifold and being split into a plurality of parts to each other by adhesion or fusion to obtain the manifold with the predetermined shape. Further, as shown in FIGS. 19, 20, a fifth method involves the steps of: molding internal molding elements 119 having a predetermined shape of a hollow portion of a manifold; combining these elements together to form an internal structural body 121; and integrating an external molding 123 over the outer circumference of the internal structural body 121 by injection molding to obtain the manifold with the predetermined shape.
The aforementioned conventional constructions have addressed the following problems. First, in the first method the core 101 made of the low melting point alloy is heavy and therefore hard to handle, and the process of melting the core made of the low melting point alloy is cumbersome as well. In the second and third methods pressure is applied during injection molding to deform the internal molding, which makes it necessary to give some special measure such as reinforcement.
Further, in the fourth and fifth methods asperities are formed on the joining surfaces for adhesion, which results in defects on the inner surface of the hollow portion of the manifold, although there is no problem of injection molding.